1. Field of the Invention
The present invention pertains to the commercial manufacture of polyester polymers, in particular, polyethylene terephthalate (“PET”) polymers.
2. Background Art
PET has numerous uses, principle among which are for films, fibers, and food containers. Despite the stringent matrix of properties required for such uses, particularly for food packaging, PET has become a commodity polymer. Commercial production of PET is energy intensive, and therefore even relatively small improvements in energy consumption are of considerable commercial value.
The production of PET (inclusive of copolymers) begins with an esterification step where the dicarboxylic acid component, predominantly terephthalic acid, is slurried in ethylene glycol and heated to produce a mixture of oligomers of a low degree of polymerization. This “esterification” step may be followed by a further “oligomerization” or “prepolymer” step, where a higher degree of polymerization is obtained. The product still has a very low molecular weight at this stage.
The previously described steps are then followed by a polycondensation. The polycondensation is catalyzed by metal compounds such as Sb, Ti, Ge, Sn, etc. Polycondensation occurs at relatively high temperature, generally in the range of 280–300° C., under vacuum, water and ethylene glycol produced by the condensation being removed. The polymer at the end of polycondensation has an inherent viscosity generally in the range of 0.4 to 0.65, corresponding to a molecular weight too low for many applications.
Commercial production of PET and other polyesters as well has required a subsequent post-polymerization in the solid state, termed “solid stating.” In this stage of the process, polyester pellets are heated in inert gas, preferably nitrogen, in a solid state polymerization reactor, often termed a “solid stating reactor” or “solid stater”, at temperatures below the melt temperature, i.e. from 210–220° C. in the case of PET. Solid stating is complicated by the fact that most PET polymers and other polyesters as well, following extrusion from the melt and pelletizing, are substantially amorphous. In order to prevent the pellets from sintering and agglomerating in the solid stater, the pellets are first crystallized over a period of 30 to 90 minutes at a lower temperature, e.g. 160–190° C., typically in a flow of inert gas. It should be noted that “solid stating” herein refers to the solid state polycondensation per se, and not to the combined processes of crystallization and solid state polycondensation.
Following polycondensation in the solid state, it has been the practice to cool the pellets in a stream of cool air or nitrogen gas, which is then cooled and recycled. Considerable quantities of gas are required, as well as circulation pumps of large capacity. Moreover, the equipment required for cooling is large, and thus capital intensive. Use of water for cooling is not known, most likely because it had been thought that water associated with water-cooled pellets required complete removal. as otherwise it may cause polymer hydrolysis during processing steps such as extrusion and injection molding. For these reasons, PET pellets are thoroughly dried before use.
In U.S. published application 2003/0039594 A1, a method is disclosed for cooling hot polymer pellets from a solid stating reactor where a conventional fluidized bed cooler is used, but augmented by water spray into the cooler proximate the hot pellet inlet. A first section of cooler is isolated from a further section, the first section operating minimally at 230° F. to avoid overwetting of pellets. The object of the '594 publication is to utilize the heat of vaporization of water to aid in pellet cooling, while also reducing the flow of gas to the cooler. However, by spraying water over the bed in the heated chamber, considerable water is vaporized by contact with hot gas rather than hot pellets, and when a closed gas recirculation system is employed, a dehumidifier must be added to the gas recirculation line. Not only does the process of the '594 publication involve only a modest improvement in energy usage in the pellet cooling process, it moreover requires monitoring and adjustment of additional parameters in the fluidized bed cooling unit.
It would be desirable to provide a process for cooling pellets which does not require a large volume air stream, and yet which provides pellets which are suitable for later processing by conventional molding technology such as injection molding.